The invention relates to an optical system and a method for spectroscopy. More particularly, the present invention relates to an optical system and a method for Raman spectroscopy.
Due to its suitability for in-situ (on site) and online analysis, the Raman spectroscopy is increasingly used in industrial process and environmental measurement technology. A disadvantage in this case, however, is that some samples provide spectra, in practice due to fluorescence and/or due to their scattering properties, whose quality is impaired by a high-energy subsurface, in particular when worked with measuring times with less than 1 s, without sample preparation and/or with small-sized apparatuses.
Typically, stationary samples are used for recording Raman spectra, in which the Raman radiation generated can be integrated over a sufficiently long measuring period for the suppression of a measuring subsurface. In this case, excitation radiation of a fixed excitation wavelength is coupled into a sample to be examined and is inelastically scattered by the sample. The scattered excitation radiation can then be collected by a suitable spectroscopy arrangement and spectrally examined. Depending on the specific material properties of a sample, individual Raman lines show in the recorded spectrum which have a precisely determined spectral distance from the excitation wavelength of the excitation radiation, which is characteristic for the type of the sample.
However, such a setup is usually not suitable for recording a Raman spectrum of a moving sample because, in this case, both the excitation of the sample and the detection of an excitation radiation scattered thereon must be directed after the current position of the sample. A continuous recording of a complete Raman spectrum by a conventional spectrometer is often difficult to realize because of the integration and measurement times which are typically in the range of several tens of seconds. In particular, the measured signal intensities can be subject to considerable fluctuations due to a movement of the sample, so that generally no valid statements about the intensities of individual Raman lines with respect to one another can be obtained.
If, on the other hand, the samples to be examined are moved separately and along a fixed measuring distance, the requirements for a spectroscopic setup can be considerably reduced. For example, in the flow cytometry, individual sample elements (for example, particles, cells, viruses, microspheres) can be specifically excited and examined for their specific property. However, so far primarily fluorescence examinations have been carried out, since in a conventional flow cytometry the recording of a Raman spectrum is usually not possible due to a relatively short observation time per sample element.
It is therefore an object of the present invention to provide an optical system and a method for spectroscopy, in particular for Raman spectroscopy, with which also individual moving sample elements can be examined spectroscopically, which are not based on the complex spectroscopy setup known from the prior art, and which can be implemented with low-priced components and which are suitable for the setup of particularly small, compact and robust arrangements.